CN103868835 (A) discloses a system for monitoring PM2.5 by a smart phone. The system is provided with a small and intelligent air sampling head. The intelligent air sampling head can perform inhaling sampling through a human mouth and also can perform sampling by a sampling air pump. The intelligent air sampling head and the smart phone can communicate with each other. The intelligent air sampling head measures the number and particle size of dust in air by a laser scattering method under the guide of the smart phone. The system is complicated and expensive.
US 2013/0120759 A1 discloses an apparatus for measuring a distance. A self-mixing interference (SMI) unit generates an SMI signal, wherein the SMI unit comprises a laser emitting a first laser beam for being directed to an object and wherein the SMI signal depends on an interference of the first laser beam and a second laser beam reflected by the object. A peak width determination unit determines a peak width of the SMI signal, and a distance determination unit determines a distance between the object and the SMI unit depending on the determined peak width of the SMI signal.
US 2015/0077735 A1 discloses a device for determining wind speed comprises at least two laser sources emitting beams in different directions that are coplanar and such that each emission direction corresponds to a perpendicular emission direction. Each laser source is associated with focusing optics for focusing the emitted beam, a laser diode for receiving a reflected beam obtained after reflection by a particle present in the air of the corresponding emitted beam, a photodiode for transmitting an interference signal occurring between the emitted beam and the reflected beam, a processor for processing the obtained interference signals, and an optical cavity into which the reflected beam is reinjected in order to obtain an interference with the emitted beam.
US 2013/0215411 A1 discloses a laser Doppler velocimeter using self-mixing amplification from backreflections from scatterers below a surface of a flow.
WO 2014/167175 A1 discloses a velocimeter which comprises a laser source comprising a laser cavity for producing a continuous-wave laser beam, a light detector, and means for directing a first portion of the laser beam to a moving target outside the velocimeter for producing scattered light from the target, the scattered light exhibiting a Doppler shift corresponding to the speed of the moving target. Further, the velocimeter comprises means for guiding scattered light back to the laser cavity for providing frequency-modulation of the laser beam through perturbance in the laser source, and means for guiding a second portion of the laser beam to the light detector. There is further provided an optical edge filter adapted to demodulate said frequency-modulated laser beam before guiding to the light detector and wherein said laser source is adapted to produce laser light at a wavelength corresponding to the wavelength of the edge of the optical edge filter.